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1991
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l3.asm
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Assembly Source File
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1991-11-11
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7KB
|
193 lines
; QSCAN3.ASM
; David Stafford
; 8/23/91
COMMENT $
How it works
------------
The idea is to go through the buffer fetching each letter-pair (words
rather than bytes). The carry flag indicates whether we are
currently in a (text)word or not. The letter-pair fetched from the
buffer is converted to a 16-bit address by shifting it left one bit
(losing the high bit of the second character) and putting the carry
flag in the low bit. The high bit of the count register is set to
1. Then the count register is added to the byte found at the given
address in a large (64k, naturally) table. The byte at the given
address will contain a 1 in the high bit if the last character of the
letter-pair is a word-letter (alphanumeric or apostrophe). This will
set the carry flag since the high bit of the count register is also a
1. The low bit of the byte found at the given address will be one if
the second character of the previous letter-pair was a word-letter
and the first character of this letter-pair is not a word-letter. It
will also be 1 if the first character of this letter-pair is a
word-letter but the second character is not. This process is
repeated. Finally, the carry flag is saved to indicate the final
in-a-word/not-in-a-word status. The count register is masked to
remove the high bit and the count of words remains in the count
register.
Sound complicated? You're right! But it's fast!
The beauty of this method is that no jumps are required, the
operations are fast, it requires only one table and the process can
be repeated (unrolled) many times. QSCAN3 can read 256 bytes without
jumping.
COMMEND $
.model small
.code
Test1 macro x,y ;9 or 10 bytes
Addr&x: mov di,[bp+y] ;3 or 4 bytes
adc di,di
or ax,si
add al,[di]
endm
Test2 macro x,y ;7 or 8 bytes
Addr&x: mov di,[bp+y] ;3 or 4 bytes
adc di,di
add ah,[di]
endm
Scan = 128 ;scan 256 bytes at a time
Buffer = 4 ;parms
BufferLength = 6
CharFlag = 8
WordCount = 10
public _ScanBuffer
_ScanBuffer proc near
push bp
mov bp,sp
push si
push di
xor cx,cx
mov si,[bp+Buffer] ;si = text buffer
mov ax,[bp+BufferLength] ;dx = length in bytes
shr ax,1 ;dx = length in words
jnz NormalBuf
OneByteBuf:
mov ax,seg WordTable
mov es,ax
mov di,[bp+CharFlag]
mov bh,[di] ;bh = old CharFlag
mov bl,[si] ;bl = character
add bh,'A'-1 ;make bh into character
add bx,bx ;prepare to index
mov al,es:[bx]
cbw ;get hi bit in ah (then bh)
shr al,1 ;get low bit
adc cx,cx ;cx = 0 or 1
xchg ax,bx
jmp CleanUp
NormalBuf:
push bp ;(1)
pushf ;(2)
cwd ;dx = 0
mov cl,Scan
div cx
or dx,dx ;remainder?
jz StartAtTheTop ;nope, do the whole banana
sub cx,dx
sub si,cx ;adjust buf pointer
sub si,cx
inc ax ;adjust for partial read
StartAtTheTop: mov bx,dx ;get index for start...
shl bx,1
mov di,LoopEntry[bx] ;...address in di
xchg dx,ax ;dx is the loop counter
xor cx,cx ;total word count
mov bx,[bp+CharFlag]
mov bl,[bx] ;bl = old CharFlag
mov bp,seg WordTable
mov ds,bp
mov bp,si ;scan buffer with bp
mov si,8080h ;hi bits
mov ax,si ;init local word counter
shr bl,1 ;carry = old CharFlag
jmp di
align 2
Top: add bx,bx ;restore carry
n = 0
rept Scan/2
Test1 %n,%n*2
Test2 %n+1,%n*2+2
n = n+2
endm
EndCount:
sbb bx,bx ;save carry
if Scan ge 128 ;because al+ah may equal 128!
or ax,si
add al,ah
mov ah,0
else
add al,ah
and ax,7fh ;mask
endif
add cx,ax ;update word count
mov ax,si
add bp,Scan*2
dec dx ;any left?
jng Quit
jmp Top
Quit: popf ;(2) even or odd buffer?
jnc ItsEven
clc
Test1 Odd,-1
sbb bx,bx ;save carry
shr ax,1
adc cx,0
ItsEven:
push ss ;restore ds
pop ds
pop bp ;(1)
CleanUp:
mov si,[bp+WordCount]
add [si],cx
adc word ptr [si+2],0
and bh,1 ;save only the carry flag
mov si,[bp+CharFlag]
mov [si],bh
pop di
pop si
pop bp
ret
_ScanBuffer endp
.data
Address macro X
dw Addr&X
endm
LoopEntry label word
n = Scan
REPT Scan
Address %n MOD Scan
n = n - 1
ENDM
.fardata WordTable
include qscan3.inc ;built by MAKETAB
end